Receivers in global navigation satellite systems (GNSS's), such as the Global Positioning System (GPS), use range measurements that are based on line-of-sight signals broadcast by satellites. The receivers measure the time-of-arrival of one or more of the broadcast signals. This time-of-arrival measurement includes a time measurement based upon a coarse acquisition coded portion of a signal, called pseudo-range, and a phase measurement.
In GPS, signals broadcast by the satellites have frequencies that are in one or several frequency bands, including an L1 band (1565 to 1585 MHz), an L2 band (1217 to 1237 MHz), an L5 band (1164 to 1189 MHz) and L-band communications (1525 to 1560 MHz). Other GNSS's broadcast signals in similar frequency bands. In order to receive one or more of the broadcast signals, receivers in GNSS's often have multiple antennas corresponding to the frequency bands of the signals broadcast by the satellites. Multiple antennas, and the related front-end electronics, add to the complexity and expense of receivers in GNSS's. In addition, the use of multiple antennas that are physically displaced with respect to one another may degrade the accuracy of the range measurements, and thus the position fix, determined by the receiver. Further, in automotive, agricultural, and industrial applications it is desirable to have a compact, rugged navigation receiver. Such a compact and rugged receiver may be mounted inside or outside a vehicle, depending on the application.
The ideal antenna for the reception of signals from GPS satellites would have a gain of 3 dB isotropic for the upper hemisphere, which sees the sky, and no gain for the lower hemisphere, which sees the earth. Additionally it would have a polarization of right hand circular (RHCP). In recent years other GNSS have supplemented the GPS signals, and their signals are best received with the same gain pattern and polarization of the ideal GPS antenna. Sometimes the accuracy of these GNSS signals are enhanced with differential corrections generated by reference receivers and transmitted on satellite downlinks at frequencies slightly lower than GPS L1. Fortunately these correction signals are also RHCP, but they tend to be of lower power and are available from fewer satellites than the GNSS signals. All together, these GNSS and communication bands cover from 1150 MHz to 1610 MHz in frequency.
Various attempts to receive all of these frequencies with an RHCP antenna having the desired gain pattern, and a moderate cost and size have been made. Most of these have gain patterns which are quite good at high elevation angles (i.e. close to straight up), but drop rapidly closer to the horizon.
There is a need, therefore, for improved compact antennas for use in receivers in GNSS's to address the problems associated with existing antennas.